Multiple shell structures for use as laser fusion targets have been proposed previously. FIGS. 4 and 5 of Woerner U.S. Pat. No. 4,154,868, for example, show multiple shell targets wherein an inner glass shell is supported (by means not shown) concentrically within an outer glass shell, with the region between the shells being evacuated. It has been proposed that the required structure for supporting the inner shell (not shown in the Woerner patent) may comprise a fiber extending between the inner surface of the outer shell through the intervening space to the outer surface of the inner shell.
A second type of multiple shell target which has been proposed is one wherein the inner shell is nested within but unsupported by the outer shell. This type of structure, to which the present invention is directed, is one wherein the inner shell may be said to "float" with respect to the outer shell. As used in the present application, the term "float" is not limited to a connotation of buoyancy with respect to the outer shell, but is intended in the broader sense to mean that the inner shell is unsupported by--i.e. is not structurally rigidly coupled to--the outer shell. If the inner shell is more dense than the material in the region between the shell walls, which may be gaseous for example, or if the intrashell is evacuated, the inner shell would normally rest by force of gravity on the lowermost portion of the surrounding outer shell.
One method previously proposed for forming the nested shells involves formation of the outer shell as two hemishells and assembly of the hemishells with the inner shell contained therein. The material inhomogeneity at the resulting seam in the outer shell is undesirable for ICF targets. Another method, termed the soluble or disposable mandrel technique, involves encapsulation of the inner shell in a soluble material, followed by formation of the outer shell around the encapsulant. An opening is then formed in the outer shell and the encapsulant is dissolved away. The hole in the outer shell, which must then be plugged, results in a small but appreciable non-uniformity in the outer shell wall. An additional problem is the expense that would be involved in application of either technique to production of nested shells in quantity.